The objective of this grant is to study the effect of large oral doses of vitamin B6, initiated in early infancy, upon cranial circumference and psychometric testing in patients with the trisomy 21 form of Down's syndrome. A double blind study comparing 25 mg./kg of vitamin B6 with placebo will be initiated in the neonatal period and continued until 3 years of age. In vivo, PLP and 4PA levels will be obtained in double blind patients. In vitro studies of PLP in whole blood using the red cell model system, will also be completed in older patients with Down's syndrome. Any side effects that occur from the pharmacological doses of vitamin B6 given to these patients will be recorded.